In the field of communications, it is known that at least three different physical (PHY) layers for the IEEE 802.11 WLAN are available (IEEE 802.11a/b/g), and, they all provide multi-rate capabilities for devices. To achieve a high performance under varying conditions, these devices need to adapt their transmission rate dynamically. While the rate adaptation feature is a critical component of their performance, only very few algorithms have been published and the implementation challenges associated with these published algorithms have not been totally resolved.
Reducing the energy consumed by wireless communication devices is perhaps the most important issue in the widely-deployed and exponentially-growing IEEE 802.11 Wireless LANs (WLANs) applications. TPC (Transmit Power Control) and PHY (Physical layer) rate adaptation have been recognized as two most effective ways to achieve this goal. The emerging IEEE 802.11h standard, which is an extension to the current IEEE 802.11 MAC and the high-speed IEEE 802.11a PHY, will provide a structured means to support intelligent TPC. It has been proposed to minimize the communication energy consumption in IEEE 802.11a/h systems by combining TPC with PHY rate adaptation. In one approach, the key idea is to compute offline an optimal rate-power combination table, and then at runtime, a wireless station determines the most energy-efficient transmission strategy for each data frame by a simple table lookup. Known art also provides a rigorous analysis of the relation among different radio ranges and the effect of TPC on the interference in IEEE 802.11a/h systems, which justifies an approach to ameliorating the TPC-caused interference by transmitting the CTS (Clear To Send) frames at a stronger power level. The approach in known art delivers about 20% more data per unit of energy consumption than the PHY rate adaptation scheme without TPC, while outperforming single-rate TPC schemes significantly, owing to the energy-saving capability of PHY rate adaptation.
The wireless medium is known to be associated with certain issues and conditions which reduce the reliability of the medium. Notwithstanding, it is also known that the wireless medium provides mobility, which accommodates variation in the distance between transmitter and the receiver. Presence of objects/obstacles in the medium between the transmitter and the receiver causes reflection of radio waves from these objects, leading to scattering, multi-path and fading due to which the performance degrades. In the case of applications based on the IEEE 802.11 WLAN specifications but using the unlicensed spectrum, performance-degradation may be observed because of interference from other devices operating in the same frequency spectrum. Presence of other objects in the medium between the transmitter and the receiver causes reflection of radio waves from these objects leading to scattering, multi-path and fading due to which the performance degrades. A good measure of the wireless network performance is the throughput that is made available to the application. Performance degradation reduces the throughput available to the application and is not desirable for applications which demand reasonable and reliable throughput, especially for VoIP type applications.
Various methods of approach have been addressed for link adaptation purposes as exemplified by the following U.S. Pat. No. 6,122,293, (Method and System for Link Adaptation having a variable update interval); U.S. Pat. No. 6,289,217 (Adaptive radio Link); U.S. Pat. No. 6,385,462, (Method and System for criterion based adaptive power allocation in a communication system with selective determination of modulation and coding); U.S. Pat. No. 6,374,117 (Queue based Power control Scheduling); U.S. Pat. No. 6,643,322 (dynamic Wireless Link Adaptation); U.S. Pat. No. 6,728,259 (Link Adaptation Algorithm for Packet based Radio System); and, U.S. Pat. No. 6,760,596 (method and System for bit Rate Adaptation to improve coverage).
However, available methods of transmission and control through link adaptation may sometimes not be suitable for especially VoIP applications which demand power conservation, performance and reliable throughput. It is noted in this context that the use of PER (Packet Error Rate) as a measure of link quality, as done in several prior art approaches is not wholly suitable for VoIP scenarios, since it requires multiple packet transmissions to obtain a statistically valid PER, whereas voice packets in VoIP are less frequent to support generating a valid PER. Also, the use of PER as a measure of link quality is not conductive to conserving the power resource in battery operated units, considering that the payload is small for a VoIP packet, and it is more essential to conserve power rather than maximize throughput.
There is therefore a need to make an innovation in the area of especially VoIP applications in order to enhance efficiency and conserve power in the approach to link adaptation.